This present invention relates to improved methods and apparatus for delignifying cellulosic material in modern displacement batch digester systems.
In conventional batch pulping operations, the wood material (softwoods, eucalyptus, or hardwoods) or the lignocellulosic crop material (e.g., bagasse, bamboo, kenaf, reeds, and so forth) is reacted with cooking liquor for a given time at a specified temperature. The cooking chemistry may be kraft (sulfate), sulfite or other.
Conventional batch reactors (digesters) used for producing fibrous cellulose from fiber bearing sources, such as wood, have traditionally been filled simultaneously with the wood chips and cooking chemicals required to liberate the cellulosic fiber. Recently, new processes have emerged that reuse the spent cooking liquor that have been displaced from previous batch cooks (spent liquor) to save energy and to take advantage of the residual chemicals in the spent liquor. These new types of cooking systems require that the stored spent liquor is pumped into the digester for reuse, referred to as the displacing liquor, to replace the liquor inside the digester, referred to as the displaced liquor, with the displaced liquor exiting the digester. Hence, they are referred to as Displacement Batch Digesters.
In order for all the contents of the digester to have the same chemical activity and to produce consistent quality pulp, the liquor that is pumped into of the digester must flow in a plug flow (i.e. no channeling) profile. The fluid dynamics of this type of arrangement favors the liquors to follow the path of least resistance which is to flow against the smooth outer digester walls or to force an open channel through the chip bed in the digester instead of flowing uniformly through the chip bed. Therefore, the fluid tends not to form a plug flow profile and the contents of the digester are not exposed to the same chemicals and temperatures.
The liquor inside the digester is replaced in phases or steps. A distinct step is defined by the specific source of the displacing liquor that is pumped to the digester. A different step infers that different temperature liquor from a distinct source vessel (tank or pressure vessel) is displacing the digester contents. The different temperature displacing liquors from the tank farm initially are used to heat the digester, and after the high temperature reaction phase of the cooking cycle is completed, to cool the digester. At the start of the cooking cycle, previously stored spent cooking liquors hotter than the temperature of the digester contents are the displacing liquors so as to impart the thermal and chemical energy to the digester's cellulosic feed material, such as wood. At the end of the cooking cycle after the high temperature reaction phase is completed, cool washer filtrate is the displacing liquor and the digester's hot spent liquor, the displaced liquor, is displaced from the hot digester and stored in the appropriate tank farm vessel for reuse in subsequent cooking cycles.
The reuse of the hot spent cooking liquor requires a tank farm composed of atmospheric tanks as well as pressure vessels to store displaced liquors over 100° C. The pressure vessels are referred to as accumulators since they accumulate the displaced return liquors at temperatures above atmospheric flash points from the digester for reuse. The displaced liquors are segregated by temperature in the tank farm with the coolest displaced liquor under around 95° C. from the digester being diverted to the atmospheric tank. The liquors above around 95° C. displaced from the digester are diverted to the first lowest temperature accumulator with the next hottest displaced liquors diverted to one or more higher temperature accumulator(s). Note that the temperatures of the liquor deposited in to each receiving vessel is actually a range of temperatures since the displacing liquor, which later becomes the displaced liquor, is exchanging thermal energy with the digester contents over the period of the displacing operation.
U.S. Pat. Nos. 4,814,042; 6,719,878; 5,800,674; 4,578,149; 4,601,787; 6,103,057; 5,059,284; 5,080,757; 4,849,052; 6,139,689; 4,764,251; 4,670,098; 4,764,251; H1,681; 4,764,251; 6,306,252; 6,346,166; 6,346,167; 6,350,348; 6,391,628; 6,451,172 and 6,514,380 disclose various aspects for the delignification (cooking) process of displacement batch digesters for providing cellulosic pulp.